1. Field of the Invention
The present invention relates to a method for, in a moving image processing apparatus which displays moving images, improving gradation correction during low illuminance and low luminance, by giving consideration to human visual features in a dark place.
2. Description of the Related Art
Conventionally, to improve image quality and visibility, a known technology is to adaptively control the contrast and brightness of a video signal and the display luminance and color signals of a display device based on the display contents of an image signal and the brightness of the surrounding environment of the viewer.
For example, Japanese Patent Application Laid-Open No. 06-83287 discusses a method for improving visibility by automatically controlling both or only one of the amplitude and an operation reference level of an applied video signal to a liquid crystal panel, based on a brightness control signal from an ambient light sensor or the like.
Further, Japanese Patent Application Laid-Open No. 2007-248936 discusses a method for improving image quality and visibility by performing image correction by giving consideration to various image characteristics, even for when the input signal is a computer graphics (CG) image or natural moving images on television.
In addition, Japanese Patent Application Laid-Open No. 2006-039520 discusses a method for maintaining color purity from low luminance to high luminance by controlling the amount of light from a light source (backlight) based on the brightness of an image signal to reduce color tone change between gradations.
However, for a conventional moving image processing apparatus, in an environment in which the illumination is turned down, when images having a low average luminance, such as those in a movie, are viewed, the color tone and gradation are perceived to gradually change, because not enough consideration is given to the human visual features in a dark place. This is because in order for a human sensory system to deal with various environments, the sensory system has a function that changes the sensitivity to that environment. For example, when a person first enters a movie theater while a movie is being played, the theater appears pitch dark and the person cannot see anything. However, after some time has elapsed, the person can see his/her surroundings quite well. This is because the person's sensitivity to light has changed. This phenomenon is called “dark adaptation”. Although the time required for dark adaptation differs between the retina center and peripheral vision, the discernible luminance at 5° from the retinal periphery decreases over time. After about 30 minutes, the discernible luminance becomes constant. Conversely, during switching from a dark place to a bright place, the discernible luminance becomes constant in about 0.5 seconds.
Further, during switching from a bright image to a dark image, the observer senses a blue hue over time, and conversely, during switching from a dark image to a bright image, the observer senses a red hue over time. This phenomenon is called “Purkinje shift”, in which the sensitivity of the human eye tends to shift from red to blue as the brightness becomes darker. Consequently, for a moving image processing apparatus whose white balance was set using a bright image, during switching from a bright image to a dark image, the image is perceived over time as a bluish image with an unbalanced white balance.
In addition, during switching from a bright environment to a dark environment, the perceived gradation characteristics of the human eye soften (look brighter), while during switching from a dark environment to a bright environment, the perceived gradation characteristics of the human eye harden (look darker). This phenomenon is called the “Bartleson-Breneman effect”, in which the human eye tends to sense an identical light stimulus more brightly if the periphery is darkened. Consequently, for a moving image processing apparatus whose white balance was set using a bright image, during switching from a bright environment to a dark environment, the image is perceived over time as an image with an unbalanced gradation balance.
Since the liquid crystal display control apparatus discussed in Japanese Patent Application Laid-Open No. 06-83287 is directed only to improving visibility in bright places, this apparatus does not provide any improvements for visibility or image quality in dark places.
Although the moving image processing apparatus discussed in Japanese Patent Application Laid-Open No. 2007-248936 adjusts white balance based on a Purkinje luminosity function, since no consideration is given to the time required for adaptation, this apparatus cannot flexibly handle scene changes or changes in the observation environment.
Further, similarly, the method discussed in Japanese Patent Application Laid-Open No. 2006-039520 also cannot flexibly handle scene changes or changes in the observation environment, because it does not give consideration to the time required for adaptation.